Over the past forty years or so there has been an increasing realisation of the need to replace fossil fuels with more secure sustainable energy sources. The new energy supply must also have low environmental impact, be highly efficient and be easy to use and cost effective to produce. To this end, solar energy is seen as one of the most promising technologies, nevertheless, the high cost of manufacturing devices that capture solar energy, including high material costs, has historically hindered its widespread use.
Every solid has its own characteristic energy-band structure which determines a wide range of electrical characteristics. Electrons are able to transition from one energy band to another, but each transition requires a specific minimum energy and the amount of energy required will be different for different materials. The electrons acquire the energy needed for the transition by absorbing either a phonon (heat) or a photon (light). The term “band gap” refers to the energy difference range in a solid where no electron states can exist, and generally means the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band. The efficiency of a material used in a solar cell under normal sunlight conditions is a function of the band gap for that material. If the band gap is too high, most daylight photons cannot be absorbed; if it is too low, then most photons have much more energy than necessary to excite electrons across the band gap, and the rest will be wasted. The Shockley-Queisser limit refers to the theoretical maximum amount of electrical energy that can be extracted per photon of incoming light and is about 1.34 eV. The focus of much of the recent solar cell work has been the quest for materials which have a band gap as close as possible to this maximum.
One class of photovoltaic materials that has attracted significant interest has been the hybrid organic-inorganic halide perovskites. Materials of this type form an ABX3 crystal structure which has been found to show a favourable band gap, a high absorption coefficient and long diffusion lengths, making such compounds ideal as an absorber in photovoltaic devices. Early examples of hybrid organic-inorganic metal halide perovskite materials are reported by Kojima, A., et al, Organometal Halide Perovskites as Visible Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 131(17), 6050-6051 (2009) and are based on methyl ammonium (MA) lead halide perovskites (CH3NH3PbI3 and CH3NH3PbBr3); the CH3NH3PbI3 material has a band gap of around 1.55 eV and addition of the bromine perovskites will typically shift the band gap to larger values. Kojima et al report that a solar energy conversion efficiency (or power energy conversion efficiency, PCE) of 3.8% can be obtained for the iodide material.
Further work described in US2013/0233377A1, details the preparation of A/M/X compounds, in which A is selected from organic cations and elements from group 1 of the periodic table, M is selected from at least groups 3, 4, 5, 13, 14 or 15 of the periodic table and X is selected from elements from groups 17 of the periodic table. Particular examples have the formula AMI3 where A is methylammonium (CH3NH3+), formamidinium (HC(NH)2)2+), methylformamidinium (H3CC(NH)2)2+) or guanidinium (C(NH)2)3+).
More recently in 2014, work has been conducted on formamidinium lead iodide perovskite, HC(NH2)2PbI3, not least because compared with the MA counterpart, formamidinium (FA) metal halide perovskite materials typically measures a narrower band gap of about 1.48 eV and hence is closer to the Shockley-Queisser limit (1.34 eV) which governs optimum solar conversion efficiency for a single junction device. Indeed, power conversion efficiencies (PCEs) of FA perovskites is reported to be over 17% (M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Prog. Photovolt. Res. Appl., 2014, 22, 701), thus providing persuasive evidence that this type of perovskite material has the potential to provide excellent photovoltaic materials. Further factors in favour of FA/Pb/halide perovskites are that they demonstrate greater thermal and moisture stability compared to the MA counterparts.
However, in common with other organic-inorganic perovskites, FA metal halide perovskites are able to exist in different crystalline structures i.e. different phases; each is stable over a different range of temperatures and only one phase exhibits suitable electronic properties. Unfortunately, the formation of these different phases becomes a significant problem to the ability to the use of these compounds when the temperature range over which the material transitions from one phase to another, is low, for example below 200° C.
The two phases of FA/metal/halide perovskite materials as shown in the crystallographic schematic diagram of FIG. 1 are:                a) Alpha, perovskite-type trigonal (P3m1) a=8.9920, c=11.0139 Å polymorph. Stable phase at temperature (T)>60° C. (black); and        b) Delta, hexagonal-type (P63mc). Wide bandgap semiconductor but chain-like structure hinders electronic transport. Stable phase at temperature (T)<50° C. (yellow).        
In the case of FA/metal/halide perovskite materials it is the black alpha phase which exhibits the suitable optical and electronic properties, thus it is desirable to prepare this phase in preference to the yellow delta phase. As a further difficulty, although phase transitions as a result of changing temperature are observed in the bulk powder materials, the same is not readily observed when the materials are formed in a thin film. In this case, the initial phase formed determines the prevailing phase, therefore it is very important to form the correct phase initially when making thin films of these materials. Another consideration is the importance of the kinetics of crystal formation being able to encourage or prevent the creation of phases that are not thermodynamically the most stable. For example, if one considers crystal formation as two steps, nucleation and then growth, it is possible to seed the nucleation of one phase in preference to the other. In thin film formation the surface on which the film is forming can play a role in seeding one phase over another.
The method used to make the perovskite material is found to have a profound effect on which crystalline phase is formed. As described, for example in G E. Eperon et al, Energy Environ. Sci. 2014, conventional formamidinium (FA) perovskites (FAPbI3) can be formed in a one-step deposition process using a 1:1 stoichiometric precursor solution of lead iodide (PbI2): formamidinium iodide (FAI) in N,N-dimethylformamide (DMF). This one-step deposition process which is followed by a thermal cure, can be performed under either ambient or inert conditions, but in neither environment is free of problems and both cause difficulty when trying selectively to prepare the alpha-phase perovskite. Firstly, under ambient conditions the one-step process favours the delta-phase, whereas a mixture of both the alpha- and delta-phases is formed when these conditions are used to make solution processed thin films. Another problem is that the delta-phase is the prominant material when the thin film product is formed on a meso-porous scaffold (e.g. zirconia or alumina structures). A yet further problem with the one-step process is that the starting materials poorly convert to the perovskite and this results in the presence of PbI2 as an impurity in the product. Unfortunately, increasing the FAI content only leads to poor formation and poor performance of the thin film due to the presence of unreacted FAI, and this material presents another difficulty in that it cannot be easily removed owing to its low volatility.
Although using the one-step process under inert atmospheres is more likely to form the alpha-phase when forming thin films, as above the use of mesoporous scaffolds, such as alumina, again preferentially causes the formation of the delta-phase.
In an alternative process, for example as described in Pellet et al Angew. Chemie 2014, Vol. 53, Issue 12, pp 3151-3157, FAPbI3 can be formed via a two-step deposition of PbI2 with subsequent immersion in the FAI halide salt solution in iso-propyl alcohol (IPA). Typically, the procedure is conducted under inert and/or dry atmospheric conditions and is followed by thermal curing. This two-step process not only has the disadvantage that it requires more than one step which increases the complexity and cost of the process, but it leads to a mixture of the alpha- and delta-phases. Also, it can lead to poor conversion of the starting materials to the perovskite leaving unconverted PbI2 within the film which along with the delta-phase will typically decrease charge transport capabilities, as well as useful spectral absorption, and therefore will exhibit poorer cell performance.